This invention relates to fibers, and fabrics made from them, of ethylene copolymers. Until now the use of polyethylene for formation of fiber strands and useful textiles has been limited by processing methods for the polyethylene resins available. We have made fibers and fabrics having particularly desirable characteristics. This was accomplished by manipulating the polymer formation process rather than by some of the more cumbersome means which have been attempted previously.
Historically, the free-radical initiated and Ziegler-Natta catalysis have been the available methods for olefin polymerization and preparation of high molecular weight olefin polymers and copolymers. In the 1940's the process using free-radical initiation was developed. This technique uses high pressures, high temperatures, and a free-radical initiator such as peroxides. When ethylene is polymerized in a free-radical initiation process, the polyethylene formed by such a process will generally have densities in the range of about 0.91-0.935 g/cm.sup.3 and is called low density polyethylene (LDPE). Polyethylene formed by the free-radical method will generally have a high level of random branching of varying length.
In the late 1950's and early 1960's the use of "Ziegler-Natta" (Z-N) catalysts became common. These catalysts are used in a wide range of processes including low, medium, and high-pressure processes. Generally, when ethylene is polymerized using a Z-N catalyst, a "linear" product will result whose polymer molecules will be substantially unbranched. Such linear polyolefins generally have relatively high densities, in the ranges of about 0.941 to about 0.965 g/ml, which result from closer packing of the polymer molecules and minimal chain entanglement compared with the more highly branched and less dense materials. One characteristic of the polymeric species produced using the Z-N catalysts is their very broad molecular weight distribution. The same phenomenon is noted with the LDPE's.
Sawyer et al, U.S. Pat. Nos. 4,830,987, 4,880,691; 4,909,975, describe the use of an ethylene/octene (E-O) copolymer with 0.919 density in some limited fiber applications. Unfortunately, with the traditional Ziegler-Natta catalysts available at the priority date of Sawyer et al, fibers of particularly low-density polyethylene were difficult to make due to the wide MWD's inherently produced in those polymers. Kubo, et al, U.S. Pat. No. 5,068,141 report difficulties in demonstrating the teachings of Sawyer, et al.
A challenge with a polymer having a broad MWD is the likely wide variation in processing among batches. A Ziegler-Natta-type broad molecular weight distribution (MWD) material will include significant fractions of molecules which are both longer and shorter than the nominal weight. The presence of those species influence the properties of the resins.
A polymer with a large fraction of short backbone chains will be very free-flowing at relatively low temperatures; but that same fraction will cause the polymer, or its products to feel sticky or tacky, have an unpleasant odor or taste, smoke during processing, and have particularly low tensile strength. The fibers may be very difficult to process due to continuous "slubbing" or breakage during attempted fiber formation. Some slubbing appears to result from formation of low-molecular-weight polymer globules on the surface of the die face or fiber as it is formed through the die. These globules may break away from the face of the fiber-forming member or impinge upon the surface of the fiber causing a break or other imperfection.
Other difficulties arise with high-molecular-weight species. A polyolefin resin having a large fraction of very long-chain polymer species for a particular nominal molecular weight will form fibers well but they will be brittle or feel particularly coarse due to a high degree of crystallinity within the polymer itself.
Kubo, et al describe, in U.S. Pat. No. 5,068,141, formation of non-woven fabrics comprising filaments formed of linear low-density polymer of ethylene and octene. They note that the range of comonomer incorporation within the polymer used to form the fabric is limited due to rigidity at low percentages of incorporation and difficulty in forming a fine filament at high percentages of comonomer incorporation. They also note that the useful range of densities of the polymers which are suitable for this application are constrained due to poor tenacity of filaments obtained at densities in which the current invention allows production of fine fibers which have acceptable tenacity and are soft and pleasing to the touch.
Fowells, U.S. Pat. No. 4,644,045, uses a polymer melt at low-temperature polymer melt for fiber spinning or drawing, to overcome processing problems in slubbing and breaking by adjusting the spinning or fabric-formation processing conditions. Kubo, et al report that this technique leads to poor drawing, which in turn leads to frequent filament breaks caused by the high tensions necessary in this low-temperature operation.
Krupp et al, U.S. Pat. No. 4,842,922, describe spunbonded fabric prepared from a blend of linear polyethylenes at high rates of production. Krupp has recognized the inherent difficulty in forming fibers and subsequent non-woven fabrics from high-molecular-weight polyethylene. The solution proposed by Krupp et al is to blend a high-molecular-weight polyethylene, particularly a linear low-density polyethylene, with low-molecular-weight polyethylene. Again, this is a cumbersome multi-step process designed to overcome processing limitations.
Coleman, U.S. Pat. No. 4,612,300, describes a polymerization catalyst which, given the then-known art of olefin catalysis, would produce a polymer with a somewhat narrowed molecular weight distribution. However, the polymer resulting here would be of relatively high density. Such material would be useful for the process of fiber formation due to high crystallinity and high melt strength but such fibers would be stiff or rigid and would yield a "boardy" fabric since actual deformation of the crystal structure must occur to provide "give" to the fabric or garment.
Kobayashi et al, U.S. Pat. No. 5,078,935, describe the production of spunbonded non-woven fabric of continuous polypropylene fibers with acceptable "hand" or tolerable feel to the touch. Unfortunately, the hand derives from an additional mechanical creping step which makes a longer fiber temporarily act as if it were shorter. Creping in this manner yields fabric from which a close-fitting garment having some "give", may be made.
Given the difficulties encountered by others and the cumbersome approaches attempted, it is apparent that it would be useful to develop polymer fibers which are soft and yielding to body movement without having to labor with the low spinning speeds required by low melt temperatures, with blending of polymers prior to spinning, or with mechanical deformation of fibers or the resulting fabric to gain a final fabric which is tolerable to the wearer. Ideally, it would be useful to form fibers which do not require such extra processing steps but yet still provide soft, comfortable fabrics which are not tacky. Our approach to solving these problems has been to use copolymers having distinctly different characteristics to produce fibers and fabrics. This strategy avoids the cumbersome processing steps described by the prior art.
A process in which the resin-maker effectively produces the "softness" of the fiber and fabric offers tremendous advantage to the fiber-maker. This moves the costly extra processing steps, such as creping or blending, out of the fiber and fabric-maker's process. This invention has done just that. Producers of fibers can simply purchase polyolefinic resins having high-value characteristics built in. Easy production of desirable fibers and fabrics with such advantages obviates the need for the less satisfactory use of cumbersome extra processing such as resin blending, post-processing creping, or slow spinning.
Until now it has been difficult to make fibers of LLDPE of the lower densities, particularly below about 0.91. These low-density polymers historically have been essentially, gooey, sticky, and formless. Attempting to draw fibers or filaments from such material would have been difficult at best. Attempts to draw fibers or filaments from such material would have led to formation of stringy strands with little tensile strength which would slub continuously or break easily and uncontrollably during formation. Any strands or fibers obtained would continue to flow, thus losing shape during further processing or use. Additionally, the processing of traditionally catalyzed polymers having low MW fractions is difficult due to "smoking" caused by volatilization of those low MW materials.
Through the practice of our invention, which makes fibers and fabrics from polymers produced by single-site catalysis, it is now possible to make fibers and fabrics with high-value characteristics where it was previously impossible. Successful fiber formation from these high molecular weight yet very low-density polyethylenes, derives from a combination of narrow molecular weight distribution and proper distribution of comonomer and ethylene throughout the backbone chain of the polymer used. This combination of properties, which derives from the formation of polymers through the use of single-site metallocene-type catalysts, allows the production of these unique fibers. Use of these distinctive fibers in textile applications provides finished textiles which are soft, "stretchy" or have a high elastic recoverability (very low permanent set), breathable, and particularly pleasant to the touch. When these textiles are used in a finished garment, they are comfortable to the wearer over long periods of time, soft and pleasant about the body, and extremely tolerant of body movement and non-restrictive in the areas of joints or other high flexion areas of the body.
With the advent and rapid recent development of single-site catalyst systems, such as those described by Welborn, EP A 129 368, Turner and Hlatky, EP A 277 003, EP A 277 004, and U.S. Pat. Nos. 5,153,157, and Canich, 5,057,475, and Canich, Hlatky, and Turner, WO 92/00333, the teachings of all of which are hereby incorporated by reference, it has become possible to more precisely tailor the molecular weight distribution of olefinic polymers as they are made. This means that remarkably narrow MWD can be obtained in materials where, only broad MWD materials existed a few years ago. LLDPE's having such a narrow MWD, yet of generally high molecular weight polymer, effectively provide polymer which does not have the low MWD fraction which causes difficulty in strand formation. These narrow MWD products have a generally higher level of crystallinity since they lack the low MW fraction. Furthermore, they are not highly crystalline materials and so are not brittle or stiff.
The result is a polymer with the remarkable ability to form new, flexible, elastic fibers from which fabrics may be easily formed. Since fibers may be formed by traditional melt spinning, spunbonding, and melt blowing, as well as extrusion, and other methods known in the art, it is now possible to form fabrics which may be stretched, draped, and worn with comfort. Such fabrics formed of these filaments, fibers, or strands have a very pleasant hand, are quite breathable, and are surprisingly lightweight. This makes garments of such fabrics quite comfortable to wear since they stretch where needed with little effort and return to their original shape immediately. Since the hand, or feel, of the fabrics is so pleasing, such garments are marvelously comfortable for the wearer. For exemplary purposes only, those garments may include diapers, particularly liners and side shields, medical gowns, as well as other single-use or disposable items. Other examples include elastic bandages, protective garments, athletic apparel including wrist and head bands, or wicking under layers, and other applications including medical drapes where elasticity and comfort are required.
Aside from creating the highly desirable characteristic of being able to tailor the molecular weight distribution of the final molecules of the polymer resin, these new metallocene-type catalysts have the desirable characteristic of being able to easily incorporate comonomers of varying size at high levels within the backbone of the polymer produced during the polymerization process. Also, as described in the previously mentioned art, these catalysts may be advantageously employed in several different polymerization processes including, for example, high pressure, medium pressure, low pressure, solution phase, bulk phase, slurry phase, and gas phase polymerization.
Molecular weight distribution (MWD) of the polymer is reported as a ratio of M.sub.w /M.sub.n. This is the weight average molecular weight divided by the number average molecular weight. MWD's in the range of about 1.8-3.5 are useful in the practice of this invention. The upper range of the apparently useful molecular weight distribution is in the area of about 3.5 but 3 or below is preferred as the upper side of the range.
The ability to incorporate comonomers at high levels within the polymer chain while still maintaining control over the narrow molecular weight distribution of the polymer appears to be unique to the single-site, particularly to the metallocene-type, catalysts. The advantageous characteristics of these new catalyst systems or catalytic compounds now makes it possible to obtain polyolefinic resins useful for forming strands or fibers, followed by incorporation of those fibers or strands into fabrics that are soft rather than "boardy", drapeable, pleasant to touch and wear, breathable, and elastic where needed, with no unpleasant odor or tackiness caused by the presence of the shorter-chain polymeric species. Additionally, these single-site catalyst-produced resins eliminate the smoking, caused by volatilization of the low MW species during processing, by not including those low MW species.
In an effort to determine a reasonable and accurate method by which distribution of comonomers throughout the polymer chain can be characterized, a new test which provides a "Solubility Distribution Breadth Index" (SDBI) has been developed. An overview of the SDBI measurement includes recognition that this is a similar measurement to the previously published "Composition Distribution Breadth Index" (CDBI), as described in WO 90/03414 which was published 5 Apr., 1990.
In general, this test provides for measurement of the solubility of a polymer resin sample at varying temperatures in a specific solvent. The net effect is that the more highly branched species within a polymer sample will be generally more soluble in solvent at the lower temperatures. As the temperature of the sample and solvent is increased, the less branched species begin to solvate. This allows for a detector, which is downstream from the elution column, to measure the amount of solvated polymer which elutes at various temperatures. From the measured solubility distribution curve, one can calculate the average dissolution temperature. One can also calculate a quantity called the solubility distribution breadth index (SDBI), which is a measure of the width of the solubility distribution curve. Through use of a fourth power term in its calculation, SDBI is defined in such a way that its value is very sensitive to the amount of polymer that is solubilized at temperatures far removed from the average dissolution temperature.
Solubility Distribution may be measured using a column which is 164 cm long and has a 1.8 cm ID (inner diameter) packed with non-porous glass beads (20-30 mesh) and immersed in a temperature programmable oil bath. The bath is stirred vigorously to minimize temperature gradients within the bath, and the bath temperature is measured using a platinum resistance thermometer. About 1.6 g of polymer is placed in a sample preparation chamber, which is repeatedly evacuated and filled with nitrogen to remove oxygen from the system. A metered volume of tetrachloroethylene solvent is then pumped into the sample preparation chamber, where it is stirred and heated under 3 atmospheres pressure at 140.degree. C. to obtain a polymer solution of about 1 percent concentration. A metered volume of this solution, 100 ml, is then pumped into the packed column thermostated at about 120.degree. C.
The polymer solution in the column is subsequently crystallized by cooling the column to 0.degree. C. at a cooling rate of about 20.degree. C./min. The column temperature is then maintained at 0.degree. C. for 25 minutes. The elution stage is then begun by pumping pure solvent, preheated to the temperature of the oil bath, through the column at a flow rate of 27 cc/min. Effluent from the column passes through a heated line to an IR detector which is used to measure the absorbance of the effluent stream. The absorbance of the polymer carbon-hydrogen stretching bands at about 2960 cm.sup.-1 serves as a continuous measure of the relative weight percent concentration of polymer in the effluent. After passing through the infrared detector the temperature of the effluent is reduced to about 110.degree. C., and the pressure is reduced to atmospheric pressure before passing the effluent stream into an automatic fraction collector. Fractions are collected in 3.degree. C. intervals. In the elution stage pure tetrachloroethylene solvent is pumped through the column at 0.degree. C. at 27 cc/min for 25 min. This flushes polymer that has not crystallized during the cooling stage out of the column so that the percent of uncrystallized polymer (i.e., the percent of polymer soluble at 0.degree. C.) can be determined from the infrared trace. The temperature is then programmed upward at a rate of 1.0.degree. C./min. to 120.degree. C. A solubility distribution curve, i.e., a plot of weight fraction of polymer solvated as a function of temperature, is thus obtained.
The procedure for calculating the Solubility Distribution Breadth Index (SDBI) is set forth below.
Solubility distributions of two ethylene copolymers are shown in FIG. 1. Here, for illustration purposes only, Sample X has a narrow solubility distribution and elutes over a narrow temperature range compared to Sample Y, which has a broad solubility distribution. A Solubility Distribution Breadth Index (SDBI) is used as a measure of the breadth of the solubility distribution curve. Let w(T) be the weight fraction of polymer eluting (dissolving) at temperature T. The average dissolution temperature, T.sub.ave, is given by: ##EQU1##
(SDBI is thus analogous to the standard deviation of the solubility distribution curve, but it involves the fourth power rather than the second power to T-T.sub.ave). Thus, for example, the narrow solubility distribution Sample X (single-site catalyst (SSC) produced) and the broad solubility distribution Sample Y (multi-site catalyst (multi) produced) in FIG. 1 have SDBI values equal to 14.6.degree. and 29.4.degree. C., respectively.
The preferred values of SDBI for fibers and fabric of this invention are less than 25.degree. C. and more preferred at less than 20.degree. C.
For the purpose of describing this invention the term "fiber" is intended to comprehend at least the litany of related terms as described by Sawyer, et al. including fiber, monofilament, multi-filament, staple, and strand, without regard to method of formation of any of these. We consider, for example, that the term "strand" will at least encompass a fiber which may be formed by normal means as well as those which may be, for example, slit or cut from a sheet or band.
The term "comonomer", for the purpose of description of this invention, is intended to comprehend at least: ethylenically unsaturated olefins or olefinic species, cyclic olefins, ethylenically unsaturated non-cyclic non-conjugated polyenes, cyclic non-conjugated polyenes, acetylenically unsaturated monomers, or combinations thereof.